Process of patterning compound semiconductor film in halogen containing etching gas

ABSTRACT

A gallium arsenide film or an aluminum gallium arsenide film is patterned through a process sequence comprising the steps of: covering the gallium arsenide film with a mask layer of indium gallium phosphide, indium gallium arsenide or indium gallium arsenic phosphide, etching a part of the mask layer in a gaseous etchant containing chlorine gas under radiating an electron beam onto the part of the mask layer for forming an etching mask, and etching a part of the gallium arsenide in the gaseous etchant, wherein one of the composition and the thickness of the mask layer is regulated in such a manner that crystal defects due to lattice mis-match are restricted, thereby preventing the gallium arsenide film from undesirable large side etching.

FIELD OF THE INVENTION

This invention relates to a patterning technology and, moreparticularly, to an electron beam lithography combined with a dryetching carried out in halogen containing etching gas.

DESCRIPTION OF THE RELATED ART

Research and development efforts are made on compound semiconductordevices such as, for example, a high electron mobility transistor or aheterojunction bipolar transistor, and patterning technology isindispensable for fabrication processes of these compound semiconductordevices. One of the attractive patterning technologies is an electronbeam lithography followed by a dry etching.

One of the dry etching techniques is disclosed by S. Miya et. al. in"Pattern Formation of GaAs using InAs/GaAs Selective Etching by Cl₂Gas", Proceedings for Fifth-Second Meeting of Applied Physics Society,page 1228, 11a-H-9. The dry etching process combined with the electronbeam lithography is hereinbelow described with reference to FIGS. 1A to1C of the drawings.

First, an indium arsenide film 1 is deposited over a gallium arsenidefilm 2 to thickness of 50 nanometer for producing a multi-levelstructure, and an electron beam 3 at 20 kilo-volts is focused on theindium arsenide film 1 as shown in FIG. 1A. An irradiated area la iscross-linked, and becomes resistive against an etchant used in the nextstage.

The multi-level structure is placed in a reactor 4 coupled with achlorine gas source 5, and the multi-level structure is heated to 190degrees in centigrade. Chlorine gas is introduced into the reactor 5 asthe etchant, and the electron beam may be continuously radiated onto thearea 1a. The chlorine gas attacks the non-irradiated area 1b of theindium arsenide, and the non-irradiated area 1b is removed from themulti-level structure. For this reason, only the irradiated area 1a isleft on the gallium arsenide film 2, and the resultant structure of thisstage is illustrated in FIG. 1B. The irradiated area 1a thus left on thegallium arsenide film 2 serves as an etching mask 1a for the galliumarsenide film 2.

Finally, using the irradiated area 1a as the etching mask 1a, thegallium arsenide film 2 is patterned. Namely, the multi-level structureis decreased in temperature to 130 degrees in centigrade, and chlorinegas pressure is regulated to 3.5×10⁻⁴ torr. The gallium arsenide film 2is exposed to the chlorine gas for 60 minutes, and the chlorine gasremoves the gallium arsenide exposed thereto. As a result, the galliumarsenide film 2 is patterned, and an area 2a overlain by the etchingmask 1a projects from the remaining area 2b by 2.7 microns.

A problem is encountered in the prior art patterning process in largeside etching due to the chlorine gas, and the prior art patterningtechnology is not appropriate for a miniature pattern.

Another problem inherent in the prior art patterning technology relatesto the etching mask 1a left on the gallium arsenide film 2. Since theindium arsenide is different in lattice constant from the galliumarsenide, crystal defects take place in the indium arsenide film 1 of 50nanometer and, accordingly, the etching mask 1a due to lattice mismatch.In order to further grow compound semiconductor films through a vacuumprocess, it is necessary to remove the etching mask 1a. However, thereis not any suitable etchant with large etch rate between indium arsenideand gallium arsenide, and the etching mask 1a is hardly removed from thegallium arsenide film 2. For this reason, the prior art patterningprocess is hardly combined with the vacuum growing process.

SUMMARY OF THE INVENTION

It is therefore an important object of the present invention to providea patterning process which is suitable for a miniature pattern as wellas combinable with a vacuum growing process.

To accomplish the object, the present invention proposes to regulate thecomposition and/or the thickness of a compound semiconductor film of anetching mask for restricting crystal defects.

In accordance with the present invention, there is provided a process ofpatterning a compound semiconductor film comprising the steps of: a)covering an objective film of first compound semiconductor selected fromthe group consisting of gallium arsenide and aluminum gallium arsenidewith a mask layer of second compound semiconductor selected from thegroup consisting of indium gallium phosphide, indium gallium arsenideand indium gallium arsenic phosphide for fabricating a multi-levelstructure, at least one of the composition and the thickness of thesecond compound semiconductor being regulated in such a manner thatcrystal defects due to lattice mis-match between the first compoundsemiconductor and the second compound semiconductor are decreased; b)etching a part of the mask layer in a gaseous etchant containing halogengas under radiating an electron beam onto the part of the mask layer forforming an etching mask exposing a part of the objective film under thepart of the mask layer; and c) etching the part of the objective film insaid gaseous etchant containing the halogen gas for patterning theobjective film.

The halogen gas may be chlorine, and the objective film may be etchedunder radiation of an electron shower fallen onto the entire surface.Moreover, the mask layer may be overlain by a protective film forpreventing the mask layer from oxygen attack.

BRIEF DESCRIPTION OF THE DRAWINGS

The features and advantages of the patterning process according to thepresent invention will be more clearly understood from the followingdescription taken in conjunction with the accompanying drawings inwhich:

FIGS. 1A to 1C are cross sectional views showing the prior artpatterning process sequence;

FIGS. 2A to 2C are cross sectional views showing a process sequence ofpatterning a compound semiconductor film according to the presentinvention;

FIGS. 3A to 3C are cross sectional views showing another processsequence of a patterning a compound semiconductor film according to thepresent invention;

FIGS. 4A to 4D are cross sectional views showing yet another processsequence of a patterning a compound semiconductor film according to thepresent invention; and

FIGS. 5A to 5D are cross sectional views showing still another processsequence of a patterning a compound semiconductor film according to thepresent invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS First Embodiment

Referring to FIG. 2A of the drawings, a patterning process embodying thepresent invention starts with preparation of an objective film 11 ofgallium arsenide grown on a substrate 12 of gallium arsenide tothickness of 1 micron. A mask layer 13 of indium gallium phosphide isgrown on the objective film 11 to thickness of 20 nanometer through amolecular beam epitaxy using gas sources as shown in FIG. 2A, and theindium content of the indium gallium phosphide is regulated to 0.48 soas to be expressed as In₀.48 Ga₀.52 P. The indium gallium phosphide thusregulated is lattice matched with the gallium arsenide, and crystaldefects are effectively decreased regardless of the thickness of themask layer 13.

Subsequently, the multi-level structure shown in FIG. 2A is placed in anetching reactor 14, and is heated to 160 degrees in centigrade. Etchinggas is introduced from a gas source 15, and the etching gas containshalogen such as chlorine. In this instance, the partial pressure ofchlorine is regulated to 5×10⁻⁵ torr, and the chlorine gas is, by way ofexample, carried by argon. Electron beam 16 is radiated from an electrongun 17 under acceleration energy at 3 kilo-volts, and is focused on apredetermined portion of the mask layer 13. In these circumstances,electron beam assisted gas etching is carried out for 5 minutes. As aresult, the irradiated portion of the mask layer 13 is selectivelyremoved from the multi-level structure, and the objective film 11 ispartially exposed. However, the nonirradiated portion 13a of the masklayer 13 is left on the objective film 11, and the nonirradiated portion13a serves as an etching mask as shown in FIG. 2B.

Subsequently, the temperature of the substrate 12 is decreased to 100degrees in centigrade, and the partial pressure of chlorine ismaintained at 5×⁻⁵ torr. The gas etching is carried out for 30 minuteswithout any electron-beam assist, and the etching gas attacks thegallium arsenide through the etching mask 13a. The objective film 11 ispartially removed as shown in FIG. 2C, and the process sequenceaccording to the present invention is available for patterning thegallium arsenide film 11.

As described hereinbefore, the composition of the indium galliumphosphide is regulated in such a manner as to be lattice matched withthe gallium arsenide, and, accordingly, crystal defects are decreased.For this reason, the etching speed are not accelerated by the crystaldefects, and the gas etching is well controlled. This results in thatthe over-etching hardly takes place, and the aspect ratio is fallenwithin a design range. The small amount of crystal defect allows asemiconductor manufacturer to carry out further crystal growth, and thepatterning process according to the present invention can be combinedwith a growing process.

In this instance, the patterned objective film 11 is formed of galliumarsenide. However, the process sequence according to the presentinvention is available for an aluminum gallium arsenide film. If themask layer 13 of indium gallium phosphide is grown over the criticalthickness of the lattice mis-match therebetween, the composition ofindium gallium phosphide should be changed in such a manner that theindium gallium phosphide is lattice matched with the aluminum galliumarsenide for restricting crystal defects. For example, if the patternedaluminum gallium arsenide is expressed as Al_(x) Ga_(1-x) As, thecomposition of indium gallium phosphide should be adjusted to In₀.48GaGa₀.52 P for lattice matching.

Moreover, if the crystal defects are effectively decreased, the problemsinherent in the prior art process are solved, and, for this reason, thethickness less than the critical thickness allows the mask layer 13 tobe lattice mis-matched with the patterned objective film 11. Forexample, if the patterned objective film 11 is formed of galliumarsenide, the critical thickness of the mask layer 13 of In₀.2 Ga₀.8 Asis of the order of 3.5 nanometer. Of course, the critical thickness isvaried with the composition of indium gallium arsenide. Similarly, ifthe patterned objective film 11 is formed of aluminum gallium arsenideexpressed as Al_(x) Ga_(1-x) As, the critical thickness for In₀.2 Ga₀.8As is also about 3.5 nanometer.

Second Embodiment

FIGS. 3A to 3C illustrate another process sequence for patterning acompound semiconductor film according to the present invention. Theprocess sequence implementing the second embodiment starts withpreparation of an objective film 21 of gallium arsenide film grown on asemi-insulating substrate 22 of gallium arsenide to thickness of 1micron. A mask layer 23 of indium gallium phosphide is grown on theobjective film 21 to thickness of 20 nanometer through a molecular beamepitaxy using gas sources as shown in FIG. 3A, and the indium content ofthe indium gallium phosphide is regulated to 0.48 so as to be expressedas In₀.48 Ga₀.52 P. The indium gallium phosphide thus regulated islattice matched with the gallium arsenide, and crystal defects areeffectively decreased regardless of the thickness of the mask layer 23.

Subsequently, the multi-level structure shown in FIG. 3A is placed in areactor 2, and is heated to 160 degrees in centigrade. Etching gas isintroduced from a gas source 25 and the etching gas contains halogensuch as chlorine. In this instance, the partial pressure of chlorine gasis regulated to 5×10⁻⁵ torr. Electron beam 26 is radiated from anelectron gun 27 under acceleration energy at 3 kilo-volts, and isfocused on a predetermined portion of the mask layer 23. In thesescircumstances, electron beam assisted gas etching is carried out for 5minutes. As a result, the irradiated portion of the mask layer 23 isselectively removed from the multi-level structure as shown in FIG. 3B,and the objective film 21 is partially exposed. However, thenonirradiated portion 23a of the mask layer 23 is left on the objectivefilm 21, and the nonirradiated portion 23a serves as an etching mask.Since electron is ten thousand times lighter than ion, the electronbombardment hardly damages the exposed portion of the objective film 21.

Subsequently, the temperature of the substrate 22 is decreased to 50degrees in centigrade, and the partial pressure of chlorine ismaintained at 5×⁻⁵ torr. The electron gun 27 radiates an electron shower28 onto the entire surface of the structure at acceleration energy of500 volts, and gas etching is carried out for 30 minutes. The etchinggas attacks the gallium arsenide through the etching mask 23a. Theobjective film 21 is partially removed as shown in FIG. 3C, and theprocess sequence according to the present invention is available forpatterning the compound semiconductor film 21.

As described hereinbefore, the composition of the indium galliumphosphide is regulated in such a manner as to be lattice matched withthe gallium arsenide, and, accordingly, crystal defects are effectivelydecreased. For this reason, the etching speed are not accelerated by thecrystal defects, and the gas etching is well controlled. This results inthat the over-etching hardly takes place, and the aspect ratio is fallenwithin a design range. The small amount of crystal defect allows asemiconductor manufacturer to further carry out a crystal growth on theetching mask 23a, and the patterning process according to the presentinvention can be combined with a growing process.

The mask layer may be formed of indium gallium arsenic phosphide In_(x)Ga_(1-x) As_(y) P_(1-y), and the composition is regulated to In₀.48Ga₀.52 As_(y) P_(1-y). The indium gallium arsenic phosphide thusregulated is available in the second process sequence instead of theindium gallium phosphide, and the indium gallium arsenic phosphide islattice latched with the gallium arsenide.

Moreover, indium gallium arsenide is further available for the masklayer. If the indium gallium arsenide is expressed as In₀.2 Ga₀.8 As,the indium gallium arsenide is lattice mis-matched with the galliumarsenide, and the mask layer is regulated under the critical thicknessof 3.5 nano-meter.

If the substance of a mask layer 23 is lattice mis-matched with thesubstance of an objective film 21, thickness of the mask layer 23 isdecreased to predetermined thickness not greater than the criticalthickness.

Third Embodiment

FIGS. 4A to 4D illustrate yet another process sequence embodying thepresent invention. The process sequence starts with preparation of asemi-insulating substrate 31 of gallium arsenide, and thesemi-insulating substrate 31 is placed in a gas-source molecular-beamepitaxial system 32. An objective film 33 of gallium arsenide is grownon the major surface of the semi-insulating substrate 31 to thickness of1 micron, and a mask layer 34 of indium gallium arsenic phosphide is inturn grown on the objective film 33 to thickness of 20 nanometer. Inthis instance, the indium gallium arsenic phosphide is expressed asIn₀.2 Ga₀.8 As₀.5 P₀.5. The indium gallium arsenic phosphide is latticematched with the gallium arsenide, and crystal defects are effectivelydecreased regardless of the thickness of the mask layer 34. A protectivefilm 35 of gallium arsenide is grown on the mask layer 34 to thicknessof 5 nanometer, and a multi-level structure is fabricated as shown inFIG. 4A.

The multi-level structure is taken out from the gas-sourcemolecular-beam epitaxial system 32, and is conveyed to an etchingreactor 36. While an operator is conveying the multi-level structurefrom the gas-source molecular-beam epitaxial system 32 to the etchingreactor 36, the multi-level structure is unintentionally exposed to theair, and undesirable natural oxide covers the surface of the multi-levelstructure.

In the etching reactor, the semi-insulating substrate 31 is heated to160 degrees in centigrade, and gaseous etchant is introduced from a gassource 37. The gaseous etchant contains chlorine, and the partialpressure of chlorine is regulated to 5×10⁻⁵ torr. Then, the protectivefilm 35 is removed together with the natural oxide, and the mask layer34 is exposed as shown in FIG. 4B.

The semi-insulating substrate 31 is maintained at 160 degrees incentigrade, and the etching gas is introduced from the gas source 37,and the partial pressure of chlorine is also maintained at 5×10⁻⁵ torr.Electron beam 38 is radiated from an electron gun 39 under accelerationenergy at 3 kilo-volts, and is focused on a predetermined portion of themask layer 34. In these circumstances, electron beam assisted gasetching is carried out for 5 minutes. As a result, the irradiatedportion of the mask layer 34 is selectively removed from the multi-levelstructure, and the objective film 33 is partially exposed. However, thenonirradiated portion 34a of the mask layer 34 is left on the objectivefilm 33 as shown in FIG. 4C, and the nonirradiated portion 34a serves asan etching mask.

Subsequently, the temperature of the semi-insulating substrate 31 isdecreased to 100 degrees in centigrade, and the partial pressure ofchlorine is maintained at 5×⁻⁵ torr. The gas etching is carried out for30 minutes without any electron beam assist, and the etching gas attacksthe gallium arsenide through the etching mask 34a. The objective film 33is partially removed as shown in FIG. 4D, and the process sequenceaccording to the present invention is available for patterning thegallium arsenide film.

The process sequence implementing the third embodiment achieves thegoals as the first embodiment, and the protective film 35 allows themulti-level structure to be exposed to the air. In detail, if noprotective film covers the mask layer of indium gallium phosphide, theindium gallium phosphide is oxidized in the air, and the natural oxidedeteriorates the reproducibility of the gas etching, because thechlorine gas can not remove the natural oxide of the indium galliumphosphide film. Another reason for the poor reproducibility is theelectron bombardment. Since electron is ten thousand times lighter thanion, the electron bombardment can not be expected to remove the naturaloxide. In order to prevent the indium gallium phosphide from oxidation,the gas-source molecular beam epitaxial system 32 should be conductedwith the etching reactor in vacuum, and the multi-level structure isconveyed in vacuum. However, such a combined system is extremelycomplex, and is dangerous because of phosphine (PH₃) used in thegas-source molecular beam epitaxial system 32.

Another approach against the natural oxide is cleaning in an arsenicatmosphere (As₄) at high temperature. However, the arsenic atoms tend toreplace the phosphorous atoms, and the indium gallium phosphide isliable to be converted into indium gallium arsenide. As describedhereinbefore, the indium gallium arsenide is lattice mis-matched withthe gallium arsenide, and a large amount of crystal defects take place.Moreover, the indium gallium arsenide film does not allow the arsenicatmosphere to further remove the natural oxide, and the cleaning in thearsenic atmosphere is not feasible. However, the protection film 35prevents the mask layer 34 from oxidation, and the natural oxide ofgallium arsenide is removed with the chlorine containing etching gas.For this reason, the protection film 35 is desirable solution, andenhances the operability in the patterning process.

Fourth Embodiment

FIGS. 5A to 5D illustrate still another process sequence embodying thepresent invention. The process sequence starts with preparation of asemi-insulating substrate 41 of gallium arsenide, and thesemi-insulating substrate 41 is placed in a gas-source molecular-beamepitaxial system 42. An objective film 43 of gallium arsenide is grownon the major surface of the semi-insulating substrate 41 to thickness of1 micron, and a mask layer 44 of indium gallium arsenic phosphide is inturn grown on the objective film 43 to thickness of 20 nanometer. Inthis instance, the indium gallium arsenic phosphide is regulated to thecomposition expressed as In₀.2 Ga₀.8 As₀.5 P₀.5. For this reason, theindium gallium arsenic phosphide is lattice matched with the galliumarsenide, and crystal defects are effectively decreased regardless ofthe thickness of the mask layer 54. A protective film 45 of galliumarsenide is grown on the mask layer 44 to thickness of 5 nanometer, anda multi-level structure is fabricated as shown in FIG. 5A.

The multi-level structure is taken out from the gas-sourcemolecular-beam epitaxial system 42, and is conveyed to an etchingreactor 46. While an operator is conveying the multi-level structurefrom the gas-source molecular-beam epitaxial system 42 to the etchingreactor 46, the multi-level structure is unintentionally exposed to theair, and undesirable natural oxide covers the surface of the multi-levelstructure.

In the etching reactor 46, the semi-insulating substrate 41 is heated to160 degrees in centigrade, and gaseous etchant is introduced from a gassource 47. The gaseous etchant contains chlorine, and the partialpressure of chlorine is regulated to 5×10⁻⁵ torr. Then, the protectivefilm 45 is removed together with the natural oxide, and the mask layer44 is exposed as shown in FIG. 5B.

The semi-insulating substrate 41 is maintained at 160 degrees incentigrade, and the etching gas is continuously introduced from the gassource 37, and the partial pressure of chlorine is also maintained at5×10⁻⁵ torr. Electron beam 48 is radiated from an electron gun 49 underacceleration energy at 3 kilo-volts, and is focused on a predeterminedportion of the mask layer 44. In these circumstances, electron beamassisted gas etching is carried out for 5 minutes. As a result, theirradiated portion of the mask layer 44 is selectively removed from themulti-level structure, and the objective film 43 is partially exposed.However, the nonirradiated portion 44a of the mask layer 44 is left onthe objective film 43 as shown in FIG. 5C, and the nonirradiated portion44a serves as an etching mask.

Subsequently, the temperature of the semi-insulating substrate 41 isdecreased to 50 degrees in centigrade, and the partial pressure ofchlorine is maintained at 5×⁻⁵ torr. The electron gun 49 radiateselectron shower 50 onto the entire surface of the structure at 500volts, and the gas etching-is carried out for 30 minutes. The etchinggas attacks the gallium arsenide through the etching mask 34a, and theobjective film 43 is partially removed as shown in FIG. 5D. However, thegallium arsenide covered with the mask layer 44a is left on thesemi-insulating substrate 41, and the process sequence according to thepresent invention is available for patterning the gallium arsenide film.

Although not repeated, the process sequence implementing the thirdembodiment achieves the goals as the first embodiment, and theprotective film 45 allows the multi-level structure to be exposed to theair.

Although particular embodiments of the present invention have been shownand described, it will be obvious to those skilled in the art thatvarious changes and modifications may be made without departing from thespirit and scope of the present invention. For example, compoundsemiconductor films may be grown through another epitaxial process, andvarious combinations of thickness and composition may be tried.Moreover, the chlorine gas may be carried by nitrogen gas.

What is claimed is:
 1. A process of patterning a compound semiconductor film comprising the steps of:a) covering an objective film of a first compound semiconductor selected from the group consisting of gallium arsenide and aluminum gallium arsenide with a mask layer of a second compound semiconductor selected from the group consisting of indium gallium phosphide, indium gallium arsenide and indium gallium arsenic phosphide to produce a multi-level structure, said second compound semiconductor having a composition lattice matched with said first compound semiconductor or said second compound semiconductor having a thickness less than a critical thickness for crystal defects; b) etching a part of said mask layer in a gaseous etchant containing halogen gas under radiating an electron beam onto said part of said mask layer for forming an etching mask exposing a part of said objective film under said part of said mask layer; and c) etching said part of said objective film in said gaseous etchant containing said halogen gas for patterning said objective film.
 2. A process as set forth in claim 1, in which said halogen gas is chlorine gas.
 3. A process as set forth in claim 2, in which said step b) is carried out under conditions where said multi-level structure is heated to 160 degrees in centigrade, the chlorine gas in said gaseous etchant is regulated to 5×10⁻⁵ torr and said electron beam is radiated under acceleration energy at 3 kilo-volts.
 4. A process as set forth in claim 3, in which said step c) is carried out under conditions where said multi-level structure is heated at 100 degrees in centigrade and said chlorine gas of said gaseous etchant is regulated to 5×10⁻⁵ torr, and no electron beam is radiated.
 5. A process as set forth in claim 1, in which the composition of said mask layer formed of indium gallium phosphide is regulated to In₀.48 Ga₀.52 P so that said indium gallium phosphide is lattice matched with said objective layer of gallium arsenide.
 6. A process as set forth in claim 2, in which said objective film is etched by said chlorine gas under radiation of electron shower onto the entire surface of the multi-level structure in said step c).
 7. A process as set forth in claim 6, in which said step b) is carried out under conditions where said multi-level structure is heated to 160 degrees in centigrade, said chlorine gas in said gaseous etchant is regulated to 5×10⁻⁵ torr and said electron beam is radiated under acceleration energy at 3 kilo-volts, and in which said step c) is carried out under conditions where said multi-level structure is heated at 50 degrees in centigrade, said chlorine gas of said gaseous etchant is regulated to 5×10⁻⁵ torr, and said electron shower is radiated at 500 volts.
 8. A process as set forth in claim 1, in which the composition of said mask layer formed of indium gallium phosphide is regulated to In₀.48 Ga₀.52 P so that said indium gallium phosphide is lattice matched with said objective layer of gallium arsenide regardless of thickness thereof.
 9. A process as set forth in claim 2, in which the composition of said mask layer formed of indium gallium arsenic phosphide is expressed as In₀.48 Ga₀.52 As₀.5 P₀.5 so that said indium gallium arsenic phosphide is lattice matched with said objective film of gallium arsenide regardless of the thickness thereof.
 10. A process as set forth in claim 2, in which the composition of said mask layer formed of indium gallium arsenide is expressed as In₀.2 Ga₀.8 As, and the thickness of the indium gallium arsenide is not greater than the critical thickness of 3.5 nanometers so that crystal defects are decreased under lattice mis-match between the indium gallium arsenide and the objective film of gallium arsenide.
 11. A process of patterning a compound semiconductor film further comprising the step of covering said mask layer with a protective film of a third compound semiconductor between said step a) and said step b), natural oxide of said third compound semiconductor being able to be removed in said gaseous etchant without any electron beam radiation.
 12. A process as set forth in claim 11, in which said natural oxide is removed in said gaseous etchant without any electron beam radiation ]prior to said etching on said mask layer in said step b).
 13. A process as set forth in claim 12, in which said third compound semiconductor is gallium arsenide.
 14. A process as set forth in claim 13, in which said natural oxide is in said gaseous etchant containing chlorine at 160 degrees in centigrade, said multilevel structure being heated to 160 degrees in centigrade.
 15. A process of patterning a compound semiconductor film comprising the steps of:a) covering an objective film of a first compound semiconductor selected from the group consisting of gallium arsenide and aluminum gallium arsenide with a mask layer of a second compound semiconductor selected from the group consisting of indium gallium phosphide, indium gallium arsenide and indium gallium arsenic phosphide to produce a multi-level structure, said second compound semiconductor having a composition lattice matched with said first compound semiconductor or said second compound semiconductor having a thickness less than a critical thickness for crystal defects; b) etching a part of said mask layer in a gaseous etchant containing halogen gas under radiating an electron beam onto said part of said mask layer for forming an etching mask exposing a part of said objective film under said part of said mask layer; and c) etching said part of said objective film in said gaseous etchant containing said halogen gas for patterning said objective film under electron flood.
 16. The process as set forth in claim 1, wherein said second compound semiconductor is protected from being exposed to oxygen by forming a protecting film thereon. 